[turner]

Closes #2117

Summary

juicebox transposes y-axis tiles, which mirrors any text baked into them. The sequence track already pre-flips its letters for this. The feature track now does the same for feature names and amino-acid letters, flipping each about its own center so it stays on its feature.

 renderFeature(feature, ..., options)
   renderAminoAcidSequence(..., sequenceInterval, options.axis)
     renderAminoAcidLetter
+      if axis === 'y': flip about the codon's center
       fillText(letter)
   renderFeatureLabel(..., options)
+    if options.axis === 'y': flip about the label's center
     fillText(name)

Evidence

New test/testRenderFeature.js drives renderFeature with a fake 2D context that tracks the transform and records each drawn glyph's anchor and horizontal scale.

feature at pixels 100-300, axis 'y'  →  label scaleX -1, anchored at 200 (center)
ATG TAA at 10 px/bp,       axis 'y'  →  "M" scaleX -1 at 18, "*" scaleX -1 at 48 (flipped about codon centers 15, 45)
no axis                              →  scaleX 1, unchanged

• Before: label is mirrored about its center on a y-axis track failed with expected 1 to equal -1, and amino acids are mirrored about their centers on a y-axis track failed with expected [ 'M', 1, 12 ] to deeply equal [ 'M', -1, 18 ].
  After: 4/4 pass, and the full suite passes (319).

Checked by eye in juicebox, with its igv dependency pointed at this branch: y-track text renders correctly.

Merge Danger

Door: two-way

A revert restores the old rendering.

Blast Radius: juicebox-only

The new code runs only when options.axis === 'y', which only juicebox sets. igv.js itself never passes axis, so its rendering and SVG export are unchanged. The flip uses only translate and scale, which canvas2svg supports.

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